1. Technical Field
The present invention relates to reinforced resin-derived carbon foams useful for high temperature and/or high strength applications, such as composite tooling; electrodes; thermal insulation; core material used in sandwich structures; impact and sound absorption; and high-temperature furnace insulation and construction. More particularly, the present invention relates to carbon fiber reinforced carbon foams exhibiting superior graphitic strength, weight and density characteristics. The invention also includes methods for the production of such carbon foams.
2. Background Art
Carbon foams have attracted considerable recent activity because of their properties of low density, coupled with either very high or low thermal conductivity. Conventionally, carbon foams are prepared via two general routes. Highly graphitizable foams have been produced by thermal treatment of mesophase pitches under high pressure. These foams tend to have high thermal and electrical conductivities. For example, in Klett, U.S. Pat. No. 6,033,506, mesophase pitch is heated while subjected to a pressure of 1000 psi to produce an open-cell foam containing interconnected cells with a size range of 90-200 microns. According to Klett, after heat treatment to 2800° C., the solid portion of the foam develops into a highly crystalline graphitic structure with an interlayer spacing of 0.366 nm. The foam is asserted to have compressive strengths greater than previous foams (3.4 MPa or 500 psi for a density of 0.53 g/cm3).
In Hardcastle et al. (U.S. Pat. No. 6,776,936), carbon foams with densities ranging from 0.68-1.5 g/cm3 are produced by heating a pitch in a mold at pressures up to 800 psi. The foam is alleged to be highly graphitizable and provide high thermal conductivity (250 W/m° K).
According to H. J. Anderson et al. in Proceedings of the 43rd International SAMPE Meeting, p. 756 (1998), carbon foam is produced from mesophase pitch followed by oxidative thermosetting and carbonization to 900° C. The foam has an open-cell structure of interconnected cells with varying shapes and with cell sizes ranging from 39 to greater than 480 microns.
Rogers et al., in Proceedings of the 45th SAMPE Conference, p. 293 (2000), describe the preparation of carbon foams from coal-based precursors by heat treatment under high pressure to produce materials with densities of 0.35-0.45 g/cm3 with compressive strengths of 2000-3000 psi (thus a strength/density ratio of about 6000 psi/(g/cm3)). These foams have an open-cell structure of interconnected cells with cell sizes up to 1000 microns. Unlike the mesophase pitch foams described above, the coal-based foams are not highly graphitizable. In a recent publication, the properties of this type of foam are described (High Performance Composites, September 2004, p. 25). The foam has a compressive strength of 800 psi at a density of 0.27 g/cm3 or a strength-to-density ratio of 3000 psi/(g/cm3).
Stiller et al. (U.S. Pat. No. 5,888,469) describe production of carbon foam by pressure heat treatment of a hydrotreated coal extract. These materials are claimed to have high compressive strengths of 600 psi for densities of 0.2-0.4 g/cm3 (strength/density ratio of 1500-3000 psi/(g/cm3)). It is suggested that these foams are stronger than those having a glassy carbon or vitreous nature that are not graphitizable.
Carbon foams can also be produced by direct carbonization of polymers or polymer precursor blends. Mitchell, in U.S. Pat. No. 3,302,999, discusses preparing carbon foams by heating a polyurethane foam at 200-255° C. in air followed by carbonization in an inert atmosphere at 900° C. These foams have densities of 0.085-0.387 g/cm3 and compressive strengths of 130 to 2040 psi (ratio of strength/density of 1529-5271 psi/(g/cm3)).
In U.S. Pat. No. 5,945,084, Droege describes the preparation of open-celled carbon foams by heat treating organic gels derived from hydroxylated benzenes and aldehydes (phenolic resin precursors). The foams have densities of 0.3-0.9 g/cm3 and are composed of small mesopores with a size range of 2 to 50 nm.
Mercuri et al. (Proceedings of the 9th Carbon Conference, p. 206 (1969)) prepare carbon foams by pyrolysis of phenolic resins. For foams with a density range of 0.1-0.4 gm/cm3, the compressive strength-to-density ratios are from 2380-6611 psi/(g/cm3). The cells are ellipsoidal in shape with cell sizes of 25-75 microns for a carbon foam with a density of 0.25 g/cm3.
Stankiewicz (U.S. Pat. No. 6,103,149) prepares carbon foams with a controlled aspect ratio range of 0.6-1.2. The patentee points out that users often require a completely isotropic foam for superior properties with an aspect ratio of 1.0 being ideal. An open-cell carbon foam is produced by impregnation of a polyurethane foam with a carbonizable resin followed by thermal curing and carbonization. The cell aspect ratio of the original polyurethane foam is thus changed from 1.3-1.4 to 0.6-1.2.
Unfortunately, carbon foams produced by the prior art processes are not effective certain applications where high thermal and electrical conductivities as well as a high compressive strength are required to maintain the structural integrity of the carbon foam. Generally, the most economical and convenient method of producing carbon foam is to directly carbonize a precursor foam derived from either phenolic or polyurethane resin. These resins are known to produce a non-graphitizable, glassy carbon, which have much lower thermal and electrical conductivities. Thus, these carbon foam structures are suitable for applications such as thermal insulation and composite tooling but not for commercial applications where higher conductivities and compressive strength are desirable.
What is desired, therefore, is a reinforced resin-derived carbon foam which is monolithic and has a controllable cell structure, where the cell structure, strength and strength-to-density ratio make the foam suitable for use in composite tooling, heat and electrical conductors, batteries and fuel cell components, aerospace components, satellite structures, cores used in sandwich structures, and also in high-temperature insulation and construction as well as in other high temperature and/or high strength applications. Indeed, a combination of characteristics, including improved conductivities and strength-to-density ratios higher than those contemplated in the prior art, have been found to be necessary for use of a carbon foam in high temperature and strength applications. Also desired is a process for preparing such foams.